The basic advantage of the human link to a system or process is man's ability to rationalize and to reach a logical decision to situations deviating from the normal. Thus, in order to employ this ability, data must be available and presented in a manner enabling the man to quickly recognize the data characteristic, compare it to known acceptable characteristics and conclude whether or not a problem exists.
Several problems exist in developing a display system that creates the desired control responses. Some of these are:
1. The ability to read the display meters with required accuracy.
2. To evaluate the readings versus acceptable values where acceptable values may vary from parameter to parameter.
3. To detect abnormal conditions quickly before the characteristic condition becomes serious.
4. Evaluate deviations from normal against the need for corrective actions.
It is obvious that a large conventional instrument panel will contain many types of meters ranging from digital displays to multipoint recorders. A man checking this panel must first establish what parameter a particular meter is reading, the value of its scale divisions and the safe limit of the parameter. This represents a considerable amount of thought and integration for each meter and a vastly larger evaluation problem where many meters exist.
The evaluation of these readings against acceptable limits is even more complex since acceptable limits vary for each parameter displayed.
The detection of abnormal conditions depends on the operator evaluating every meter reading, its relative position in the normal operating range and the rate it may be changing. Unless an operator is alert and is continuously checking the instrument panel, the probability of detecting an abnormal reading is small and may not occur until serious system trouble exists.
When deviations are detected, the need for corrective action must be determined and should be based on the parameter deviation and the rate at which it is changing.